A conventional modem allows a digital processor to communicate over a communications medium, such as an analog carrier loop. The modem converts digital signals to analog signals to be transmitted on the analog carrier loop. The modem also generates digital signals from analog signals received from the carrier loop.
The modem converts voltage levels associated with a digital signal to an analog sine wave using a modulation process. The modem transmits the modulated, analog signal on the analog carrier loop to a receiving modem. The receiving modem receives the analog signal and converts the analog signal to a digital signal using a corresponding demodulation process.
A number of modulation processes may be used to transmit digital data on an analog carrier loop. The modulation process is specified by a communications protocol (e.g., HART, FoxCom, or the Bell System Spec PUB41212) that is used to encode the data transmitted by the modem. A modem may communicate with another modem if the modems share at least one common communications protocol and both modems use the common communications protocol to transfer data.
To initiate a communication, a processor connected to the modem sends a request to the modem to send a message on the carrier loop. When ready, the modem responds to the request and the processor begins transferring data to the modem. The modem modulates the data associated with the message, bit by bit, as the data is received from the processor, to generate a corresponding analog signal. The processor must continue to transfer the data until the entire message is transmitted on the carrier loop.
A receiving modem responds to a tone of the modulated signal on the carrier loop. This tone causes the receiving modem to generate a carrier detect (CD) signal. The CD signal enables an associated processor to receive the data encoded by the modulated signal.
When transmitting, the modulator of the modem may not be able to process data as fast as the associated processor can supply the data. As a result, the processor may perform other tasks while waiting for the modulator. The modem generates an interrupt to signal when the modulator is ready to process more data. If the processor does not immediately respond to the interrupt, a delay may occur. A delay is not critical as long as the delay does not appear several times in one message. However, as the number of such delays increases, the modem may generate a timeout to cancel the message before the entire message has been received, which renders the entire message invalid.
Similar problems may occur when a message is received by a modem. If a processor associated with the modem that is receiving a message cannot respond to the interrupt of the receiving modem, the incoming data signal may overwrite a portion of the received message and result in lost data. The lost data may corrupt the entire message.
In addition to interruptions in transmission and resulting lost data, other problems may affect data communication. For example, the amplitude of the modulated analog signal also may affect communications. The amplitude of the modulated signal is reduced as the distance traveled by the signal increases. A modulated signal also is more susceptible to noise as the distance increases.
Cross coupling of signals may occur between carrier loops, communications media, and other devices in the operating environment of the carrier loop. The resulting change in amplitudes and noise on the carrier loop may be interpreted as signals by the receiving modem. The noisy signals may cause the modem to interrupt its corresponding processor, which unnecessarily burdens the processor.